Traditionally, businesses have used many different pricing models for e-commerce. For instance, eBay allows users to buy and sell items in a traditional auction. Priceline uses a pricing structure that allows sellers to compete for business by trying to meet a buyer's desired price. Other entities, such as Groupon, combine aspects of traditional coupons with group buying power to offer incentives to both buyers and sellers. In a penny auction, a user pays for the opportunity to bid on an item whose price drops with each bid.